moved ext modules docs to Whitepapers section

Author: jpeletier

mkdocs.yml

@@ -29,9 +29,9 @@ pages:
 - FAQs:
     - Lua Developer FAQ: 'lua-developer-faq.md'
     - Extension Developer FAQ: 'extn-developer-faq.md'
-    - Writing external C modules: 'modules/extmods.md'
-- Whitepapers:
-    - Filesystem on SD card: 'sdcard.md'
+    - Whitepapers:
+        - Filesystem on SD card: 'sdcard.md'
+        - Writing external C modules: 'modules/extmods.md'
 - C Modules:
     - 'adc':          'modules/adc.md'
     - 'bit':          'modules/bit.md'